1. Field of the Invention
The present invention relates to a vehicular power supply system equipped with a two-battery power supply system including a lithium-ion battery and a lead battery.
2. Description of the Related Art
Until now, there is publicly known a two-battery power supply system that includes a lithium-ion battery and a capacitor having better charge acceptability than a lead battery, stores as electric energy regenerated energy obtained during deceleration, and makes use of the stored energy in operation states other than the deceleration.
Since kinetic energy during the deceleration, which has been converted into heat by a mechanical brake and wasted, is now stored as electric power, the number of power generation times by a generator can be decreased, and as a result, a generator load imposed on the engine is reduced, so that its gas mileage can be enhanced.
In general, in using two batteries that have different nominal voltages, if they are directly connected with each other, a current flows from the one having the higher voltage of the two to the lower voltage one; therefore in order to compensate the voltage difference, a DC-to-DC voltage conversion circuit, that is, a DC-DC converter is connected between those batteries.
Moreover, a generator capable of controlling a field current to the rotor from an ECU (engine control unit) so as to control generation output is generally used, which takes control of the generation output depending on operation states, such as increasing the output during deceleration.
Although the lithium-ion battery has high energy density and excels in the charge acceptability, on the other hand, the battery can only be safely used in a limited voltage and temperature range, and in addition, it is weak in overcharge, overdischarge and also use at low and high temperatures. It is widely known that the battery would be in danger of leading to deterioration or catching fire if it is used outside a usable temperature range or repeatedly overcharged and overdischarged.
A vehicular power supply system is disclosed in Patent Document 1, for example, in which the generator and the sub power-supply thereof are connected with each other, regenerated power during deceleration is stored in the sub power-supply, and the stored power is supplied to its main power-supply and electrical load by way of a DC-DC convertor and a switch.
Moreover, another power supply system is disclosed in Patent Document 2, which includes a relay for disconnecting the sub power-supply from the power supply system, and when a lithium-ion battery that is the sub power-supply is overcharged or overdischarged, the relay is switched off to disconnect the lithium-ion battery, and power generated by the generator is supplied to its lead battery and electrical load by way of the DC-DC convertor.
Patent Document 1: Japanese Patent No. 3972906
Patent Document 2: Japanese Patent No. 4100020
However, the lithium-ion battery is charged and discharged in Patent Documents 1 and 2 even when the temperature of the battery is outside the usable temperature range, raising a fear of accelerating deterioration of the lithium-ion battery. Moreover, since the generator is usually connected to the lithium-ion battery in Patent Document 2, at the very beginning of starting generation, power is supplied from the lithium-ion battery, that is, a source outside the generator, to produce a field current and start generation, and once the generation is started, the power supply is switched over to internal power generated by the generator itself, so as to produce the field current and generate power.
Therefore, when the relay is switched off to disconnect the lithium-ion battery from the power supply system, the generator cannot produce the field current necessary for starting the generation, raising a fear of failing to generate power.
Moreover, when the generator is connected to the lithium-ion battery constituting a high voltage system as represented in the configuration of the power supplies according to Patent Documents 1 and 2, the voltage applied to the generator becomes the same as the lithium-ion battery voltage; therefore there has been another fear that a voltage generated by the generator is lowered in a state of a low engine rotation speed, such as that in idling, so that generated power would become unable to be output.
Moreover in Patent Document 2, there has been another fear that when the lithium-ion battery is disconnected from the power supply system, the relay repeats switching on and off while a current is flowing therethrough, so that contacts thereof would be welded together, accelerating deterioration of the relay.
Moreover in Patent Documents 1 and 2, when the electrical load is abruptly lost amid deceleration regeneration during which power is supplied to the lithium-ion battery and to the lead battery and the electrical load by way of the DC-DC converter, the charging amount of the lithium-ion battery abruptly increases; therefore there has been another fear that an overcurrent would flow through the lithium-ion battery or an overvoltage would be applied to the battery.
Furthermore, since output power from the generator varies depending on the temperature thereof, there has been another fear that the amount of charging current to the lithium-ion battery cannot be appropriately controlled.
Moreover, in Patent Documents 1 and 2, if the output voltage of the DC-DC converter is set to a voltage capable of charging the lead battery at all times, that is, 14.5 V, there has been another fear that power consumed in vain in the 12V-system electrical load connected in parallel with the lead battery would be increased.